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• MANUFACTURING PROCEDURE of PORTLAND CEMENT

• Two types of materials are necessary for the production of portland cements.

• - Materials rich in calcium , such as limestone, chalk • - Materials rich in silica , such as clay

• Some raw materials used in portland cement production

• Calcerous: Limestone, cement rock• Argillaceous: Clay, Shale• Siliceous: Sand, Sandstone and quarz• Ferrous : Iron ore, pyrites• Other: Gypsum, Fly ash• These raw materials are finely ground, mixed and heated in a

rotary kiln to form cement clinker.

PORTLAND CEMENT

The phase compositions in portland cement are denoted by ASTM as tricalcium silicate (C3S), dicalcium silicate (C2S), tricalcium aluminate (C3A), and tetracalcium aluminoferrite (C4AF).

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RAW MATERIALS OF CEMENT

There are four main raw materials of cements. These are;

LimestoneClaySandAdditives

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LIMESTONE

Limestone is calcareous sedimentary rocks formed at the bottom of lakes and seas with the accumulation of shells, bones and other calcium rich goods. It is composed of calcite (CaCO3). The rock which contains more than 95% of calcium carbonate is known as high-calcium limestone.

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CLAY

Clay is the second important raw material for cement. The main component of clay is aluminum silicates. Clays are plastic sediments. They consist mainly of the remains of preexisting rocks which have been broken down by weathering and erosion.

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SAND

Sand is source of SiO2. Sand is used because the ratio of SiO2 in the clay is not enough. So, sand is used as a raw material for producing cement.

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ADDITIVES

TRASS Trass is the

pozzalonic material. Trass can be found as volcanic tuff containing silica. Trass is a powerful hydraulic connector. It increases the resistance of cement. Trass contains 50-60 % silica (SiO2).

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ADDITIVES

SLAG

Slag is the product of iron-steel industry. When it is mixed with lime it shows connecting property.

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ADDITIVES

GYPSUM

Gypsum is known as calcium sulphate (CaSO4.2H2O). Gypsum is added to clinker when grinding is started. Gypsum prevents early freezing.

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PRODUCTIONPRODUCTION

Raw Material PreparationRaw Material Preparation

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Raw Grinding & Burning

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Cement Grinding & ShippingCement Grinding & Shipping

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• Cement technology is very traditional and the basic principles have remained unchanged for a long time.

• However, very profound changes in the production techniques were introduced leading to the diminishing of energy consumption and decrease in employment as well as fundamental pollution limitations.

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• Significant progress has been achieved in pollution control.

• The dust emission decreased to about 0.2-0.3 g/kg of cement.

• NOx, SO2 and CO are of the order 1200, 400 and 500 ppm, respectively, per cubic metre of kiln gases. For the special pre-calciner NOx can be as low as 400 ppm.

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• The problem is CO2 emission, which originates mainly from CaCO3 decomposition during clinker burning.

• A partial solution to this problem is the increasing production of cement with additives, chiefly slag, fly ash and limestone.

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• Raw material preparation

• Clinker produced in the properties of Portland cement is determined by the mineralogical composition of clinker.

• The classic Portland clinker has the following mineralogical composition:

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• 3CaO .SiO2 alite 55-65%

• 2CaO. SiO2 belite 15-25%

• 3CaO. AI2O3 aluminate 8-14%

• 4CaO .Al2O3 .Fe2O3 brownmillerite 8-12 %

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• The entire content of minor components such as MgO, K2O, TiO2, Mn2O, and SO3 is normally under 5 per cent.

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• Raw material crushing

• There have been no major changes in the technology of primary crushing of raw materials in recent years.

• As a rule, mainly the single- and twin-rotor hammer crushers and impact crushers are used for the primary size reduction. Jaw crushers, also in combination with roll crushers and gyratory crushers, are applied, especially for hard as well as for softer materials.

CRUSHERS

The raw materials which are taken from their mines are big pieces. But the raw materials must be broken before use. The machine which breaks the big raw materials is called crusher.

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RAW MILLS

The next step in the process is to grind the above particles to a size of 90 microns or less which is done in a raw mill, a closed circuit ball mill equipped with high efficiency separator. The main part of grinding operation is done by balls.

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• Raw material grinding

• For wet and dry grinding of the raw materials, ball mills are used which operate either as open circuit mills or in closed circuit, in case of the dry process with an air classifier. By wet grinding the process is closed through the vibrating screen or arc-screen.

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• The quick elimination of the fine fraction in closed circuit increases the grinding efficiency by about 20 per cent.

• The granulometric composition of the feed greatly influences the energy consumption.

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• The energy consumption in ball mills is also heavily influenced by the grinding media filling ratio, by the design of the mill liners and by grinding media classification in the last chamber.

• For the solution of the grinding media classification the so-called autoclassifying liners are used.

• Because of the predominance of the dry method of clinker production, in the dry grinding technology more new achievements have been introduced.

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• Classifiers

• A classifier exerts significant influence on mill efficiency and consequently on energy consumption.

• For the product containing 90 per cent of the fraction under 30 μm,the energy consumption of the ball mill is 65 kWh/t when the classifier efficiency is 25 per cent, but falls to 35 kWh/t for 100 per cent classifier efficiency.

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• Homogenization

• Raw material homogenization is a very important technological operation because the kiln feed should have a very stable chemical composition.

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• The variations of chemical composition of raw mix influence unfavourably the kiln exploitation and clinker quality.

• For a stable kiln operation, the LSF factor (lime saturation factor) fluctuations should be lower than one point. Each technological operation should increase the stability of raw materials.

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• The effect of homogenization can be calculated using the following equation:

• H= Sα/Sβ

• where Sα is the standard deviation of input variations

• and Sβ is the standard deviation of output variations.

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• There are several methods of raw meal homogenization

• Significant progress in pneumatic homogenization of raw meal has been made with continuous blending systems.

• There are two major types of blending silo designs: turbulence and controlled flow.

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• In a turbulence silo, the material is tumbled about by the injection of high volume air through air-pads on the silo floor.

• In the controlled flow system, the silo is divided into a large number of flow streams which run parallel at different flow rates, and the meal is finally blended in a tank.

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BURNING

Burning is same for all kiln systems. Clinker leaves the rotary kiln at 1400-1500°C. But before to reach that temperature there must be some processes. Also after the real burning in kiln there must be other processes to reach cold clinker. So burning process arises some common processes. They are;

Preheating (Cyclone preheaters)Precalcination (Precalciner)Rotary Kiln Clinker Cooling

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BURNING

CYCLONE PREHEATERS

Cyclone preheater kilns can have any number of stages between 1 and 6, with increasing fuel efficiency with more cyclone preheater stages. The most common is the 4-stage suspension preheater, where gases typically leave the preheater system at around 350°C.

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BURNING

PRECALCINER

In precalciner kilns, the combustion air for burning fuel in the preheater no longer passes through the kiln, but it is taken from the cooler region by a special tertiary air duct to a specially designed combustion vessel in the preheater tower. the calcination operates at temperatures around the calcination temperature of raw meal (800°C-900°C), there may not be a flame as such.

The advantages of precalciner are increasing the capacity, decreasing the energy consumption.

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BURNING

ROTARY KILNS

The real chemical reactions take place in rotary kilns. The rotary kilns are important for cement production. The main chemical reactions to produce the calcium silicates that later give cement its bonding strength occur in the kiln system.

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BURNING

ROTARY KILNS

Reactions:

Dehydration of clay minerals:

Decomposition of Carbonates:

OH4)SiO2OAl(2]OSi)OH[(Al 223210484

232232 . SiOOAlSiOOAl

23 COCaOCaCO

322322 .).(223 OAlCaOSiOCaOOAlSiOCaO

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BURNING

ROTARY KILNS

Reactions:

Solid Reactions (reactions below clinkering):

Reactions in the presence of liquid phase (clinkering):

3232 .32. OAlCaOCaOOAlCaO

32323232 ..43. OFeOAlCaOOFeCaOOAlCaO

22 .. SiOCaOCaOSiOCaO

22 .3.2 SiOCaOSiOCaOCaO

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BURNING

CLINKER COOLERSPlanetary Coolers: In

this type of cooler the flow of clinker from the kiln is divided over 10 individual cooling tubes disposed around the kiln tube.

Grate Coolers: In majority of the existing plants, conventional grate coolers are used. These coolers have lower recuperation efficiency, occupy more space and need more cooling air.

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• Clinker burning

• The dry method became predominant in the cement industry because of the much lower heat consumption, a typical value being 3000 kJ/kg of clinker for dry and 5500 kJ/kg for wet method.

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• The latest aehievement in burning technology is constituted by very short kilns with L/D ratio of about 10 .

• These kilns ensure a very rapid clinker formation, which gives a very quick reactions run without recrystallization phenomena and, as a result, a higher hydraulic activity of the clinker is obtained.

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• High volumetric loading of these kilns causes the drop in heat consumption due to a reduction of the heat losses from radiation from the kiln shell and preheater, but the main problem remains the heat losses with the exit gases.

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• Cement grinding

• For cement grinding, ball mills are more frequently used. They have a rather high-energy consumption, typically 34 kWh/t for cement having a specific surface of 300 m2/kg.

CEMENT MILLS

The clinker is cooled and ground into a fine gray powder. A small amount of gypsum is also added during the final grinding. It is now the finished product - Portland cement

During milling, the clinker, gypsum and other additives are broken down to particles less than 0.2mm in size.

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• Ball mills have low efficiency, the majority of energy being transformed into heat.

• Significant progress in grinding technology has been due to the application of roller press for pre-grinding or finish grinding.

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• The specific energy consumption for a complete cement grinding plant shown on Figure is between 26 and 29 kWh/t for a finished product.

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• Automation

• The process technology and mechanical equipment for cement production has became increasingly complex in recent years.

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• Product quality and diversity, environmental protection, secondary fuels, and energy management require more sensitive plant management.

• These are the main reasons for the introduction of computers for process control and for information systems.

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• The classical structure of the process control and information systems is distinguished by a relatively clear pyramid hierarchy.

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• Environmental protection

• The manufacture of cement necessarily involves interference with the environment, although the cement industry is doing its best to minimize the negative effects of this.

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• The problem comprises the water discharge, solid waste, noise and air pollution.

• Solid waste practically concerns kiln bricks. When chromium bricks are eliminated, the alumina bricks can be added to the raw material and magnesium bricks sent to the landfill.

• Air pollution has been the main problem for the cement industry.

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• For twenty years, a significant progress has been made within the cement industry in the field of air pollution protection.

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• The dust emission problem is now practically solved and the new construction of electrostatic precipitators and fabric filters, operating with a compressed air (reverse-pulse) cleaning system, have ensured a very low emission level, typically under 50 mg/m3 of gases.

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• Portland cements is the product obtained by pulvarizing clinker consisting essentially of hydraulic calcium silicates, usually containing one or more forms of calcium sulfate as an interground addition.

• Hydraulic calcium silicates poses the ability to harden without drying or by reaction with atmospheric carbon dioxide.

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• Five types of portland cements are recognized

• Type I : Regular portland cements are the usual products for general construction such as white (less ferric oxide), oil-well cement, quick-setting cement.

• Type II: Moderate heat of hardening and sulphate resisting portland cements are for use where moderate heat of hydration is required or for general concrete construction exposed to moderate sulphate action.

• Type III: High early strength (HES) cements are made from raw materials with lime to silica ratio higher that of Type I cements. They contain a higher proportion of tricalcium silicate (C3S) than regular portland cements. This kind of cements can be put on service road constructions insteaed of regular cements.

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• Type IV: Low heat portland cements contain a lower percentage of C3S and tricalcium aluminate (C3A). The percentage of tetracalcium aluminoferrite (C4AF) is increased because of the addition of the Fe2O3 to reduce the amount of C3A.

• Type V: Sulphate resisting portland cements are resist sulfate better than other four types. These cements are lower in C3A than regular cements. In consequence of this, the C4AF contents is higher.

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• SETTING AND HARDENING OF CEMENT

• The reaction of the setting and hardening of cement involves hydration and hydrolysis.

• The hydration products have very low solubility in water. (It is clear that the concrete is not attacted by water.)

• The heat of hardening for some compounds• C3A > C3S > C4AF > C2S

• low heat of hardening cements are made low in C3A and C3S but high C2S

• This accomplished by adding more Fe2O3, which takes Al2O3 out of circulation as C4AF, thereby diminishing the amount of C3A. and by decreasing CaO/SiO2 ratio.

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• Low heat of setting cements is used in the construction of all large dams to avoid cracking the structure from heat stresses during setting and cooling.

• Additives increases some properties of cements:

• For example; superplasticizer naphtelene derivatives

• Calcium nitrite ( to inhibit the corrosion of steel reinforcing bars in concrete)

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• OTHER CEMENTS

• For many corrosive conditions, portland cements is not suitable. For that reasons, many special cements have been developed.

• The types of special cements based on organic plastic, known as adhesives .

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• POZZOLANS:

• A pozzolans is a material which is not cementitious in itself but which becomes so upon admixture with lime.

• The early strengthof such a cement is lower than that of portland cement but within a years the strength are equal.

• The advantages: It resist the corrosive action of saline solution and seawater

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• HIGH ALUMINA CEMENTS:

• It is manufactured by fusing a mixture of limestone and bauxite ( includes also iron oxide, silica, magnesia and other impurities) and is called as calcium aluminate cements.

• The advantages: The very rapid development of strength, superior resistance to seawater and sulphate-bearing water.

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• SILICATE CEMENTS:

• This type of cements is withstand all concentration of inorganic acids except hydrofluoric.

• Using area: Joining of bricks in chromic acid reaction tanks and in alum tanks.

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• SULFUR CEMENTS

• Sulfur cements are resistanst to nonoxiding acids and salts but should not be used in the presence alkalies , oils, greases, or solvents.

• Using area: Thiokol -plasticized silica - filled sulphur cements have been accepted as a standart materials for joining bricks, and cast iron pipes.

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• POLYMER CONCRETE:

• These products consist of resins such as epoxy methyl methacrylate, or polyester.

• Advantages: rapid curing, corrosion resistance and high compressive strength

• These concretes are much higher in price than regular portland cement concrete

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• MAGNESIUM OXYCHLORIDE CEMENT:

• This cement is produced by the following reaction:

• 3MgO + MgCl2 + 11H2O 3MgO.MgCl2.11H2O

• The product is hard and strong but is attacked by water,which leaches out the magnesium chloride.

• Using area: Flooring cements with an inert filler and a coloring pignent, and as a base for such as flooring as tile and terrazzo. It is strongly corrosive to iron pipes in contact with it. Sand and wood pulp may be added as fillers.

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• The magnesia used may contain small amount of calcium oxide, calcium hydroxide, or calcium silicates , which during the setting process increase the volume changes, thus decreasing strength and durability.

• As an additive, copper powder is also added to increases water resistance , adhesion and dry and wet strenght over that of ordinary magnesium oxychloride cements.

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• LIME

• Lime itself may be used for:• - medical purposes• - insecticides• - plant and animal food• - gas adsorption• - precipitation• - causticizing

• It is employed as a reagent in the sulfite process for papermaking , dehairing hides, the manufacture of highgrade steel and cement , water softening, recovery of by-product ammonia , and manufacture of soap, rubber, varnish, refractories, and sand lime brick.

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• The suitability of lime for any particular use depends on its composition and physical properties all of which can be controlled by the selection of the limestone and the detail of the manufacturing process.

• Much lime must be finely ground before use.

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• Depending of composition,

• There are several distinct types of limes.

• - Hydraulic limes (obtained from the burning of limestone containing clay)

• - High-calcium-content limes (harden only with the adsorption of carbon dioxide from the air)- it is used especially for chemical purposes except for the sulfite paper process.

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• Magnesian limes (or dolomite) - it is used for refractory material in furnaces

HOW LIME IS PRODUCED ?

 Quarrying or mining

Limestone is extracted in both

underground and surface mines.

Lime-quality limestone must be

carefully identified and extracted to

maintain its purity.

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HOW LIME IS PRODUCED ?

Stone preparation

This step includes crushing and

screening, and in many cases,

washing to remove impurities.

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HOW LIME IS PRODUCED?

Calcining

Limestone, which is primarily composed of calcium carbonate, is heated to convert the calcium carbonate into calcium oxide. This process is generally carried out in either large rotary kilns or vertical shaft kilns. Because of the high temperatures. The product of the calcining process is quicklime, which can be used as "pebble lime," or may be crushed or pulverized, depending on its intended use.

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HOW LIME IS PRODUCED?

Hydrating

Hydrated (or slaked) lime is

produced by reacting quicklime with

water in continuous hydrators. The end

product is a fine dry powder, or, with

additional water, pumpable milk of lime. 

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Manufacturing Process of Lime

There are two main reactions when lime is produced from limestone.

Calcination

CaCO3(s) → CaO(s) + CO2(g)

Hydration

CaO(s) + H2O(l) → Ca(OH)2(s)

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Vertical Kilns

The main characteristic that all vertical kilns have in common four sections:

• Storage• Preheating• Calcining• Cooling

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Vertical Kilns

The limestone is charge into the top of the kiln

and cooled lime is discharged at the bottom.

• In the preheating; limestone is heated from

ambient to 8000C by direct contact with the gases

leaving the calcining.(i.e. Products of combustion,

excess air and CO2 from calcination)

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Vertical Kilns

• In the calcining; fuel is burned in preheated air from the

cooling and in additional combustion air added with the

fuel. This produces a temperature of over 9000C and

causes dissociation of the lime into quicklime and CO2.

• In the cooling; quicklime leaving the calcining zone at

9000C is cooled by direct contact with cooling air, part of

all of the combustion air, which in return is preheated.

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Figure 1. Vertical shaft kiln04/09/23 105Ballice, 2010

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• MANUFACTURE

• The carbonates of calcium and magnesium are obtained from deposits of limestone , marble, dolomite.

• For chemical purposes, a rather pure limestone is preferred as a starting material.

• The reactions:

• CALCINATION

• CaCO3(s) ↔ CaO (s) + CO2 H1200-1300 oC = 4 Gj/t of lime produced

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• HYDRATION

• CaO(s) + H2O (l) Ca(OH)2(s) H = -66.5 kj

• During calcination the volume contracts During hydration the volume swell

• For calcination the average fuel ratios, using bituminous coal, are 3.23 kg of lime for 1 kg coal in shaft kilns

• and 3.37 kg in rotary killns.

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• The calcination reactions is reversible. Below 650 oC the equilibrium decomposition pressure of CO2 is quite small.

• Between 650-900 oC, the decomposition pressure increases rapidly and reaches 101 kPa at about 900 oC.

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• The total heat required for calcining per ton of lime produced may be divided into two parts:

• 1. Heat to raise the rock to decomposition temperature

• 2. Latent heat of dissociation.

• Theoretical heat requirements per metric ton of lime produced.

• if the rock is heated only to a calcining temperature of 900 oC, are approximately 1.4 Gj for sensible heat and 2.7 Gj for latent heat.

• Actual calcining operations (rock is heated to 1200-1300 oC), sensible heat requirements increases by about 370 kj.

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• The practical heat requirements are approximately 4 Gj/t of lime produced in a vertical killn. About 40 percent is sensible heat, the rest is latent heat of decomposition.

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• The sequence steps connected with manufacturing in the kiln

• - Blasting down of limestone from a quarry face or occasionally from underground veins.

• - Transportation from the quarry to mills, generally by an industrial railroad.

• - Crushing and sizing of the stone in jaw and gyratory crushers

• - Screening to remove various sizes • - Carting of large stones to top of vertical

kilns• - Conveying of small stones to a rotary kiln.• - Conveying of fines to a pulvarizer to make

powdered limestone for agricultural and other demands

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• Burning of limestone according to size, (vertical kilns to give lump lime, or in horizontal rotary kilns to furnish fine lime)

• - Packaging of the finished lime in barrels or sheet-iron drums or conveying it to a hydrator.

• - Hydration of lime

• - Packing of slaked lime in 25 kg paper bags.

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• Rotary Kiln (some properties)

• -The exterior of the rotary kiln is heavy steel and interior is lined with refractory brick.

• - The kilns have a slope of 4 percent and rotate counterclockweise at a speed of 0.6 to 2 rpm.

• - Rotary kilns have the highest capacity and produce lime of the most uniform quality but required the greatest capital investment, have a high energy consumption, and need very expensive dust-collecting system.

• -Before calcination, lime is preheated to reduce fuel consumption and may be as hot as 980 oC when it enter the kiln. Heat for preheating is furnished by hot air and gases from the kiln.

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• GYPSUM

• Gypsum is hydrated calcium sulfate , with the formula CaSO4.2H2O. When heated slightly, the following reaction occurs:

• CaSO4.2H2O (s) CaSO2.1/2 H2O (s) + 3/2 H2O(g) H25 oC= 69 kj

• If the heating is at a higher temperature, gypsum loses all its water and becomes anhydrous calcium sulfate, or anhydrite.

• Calcinated gypsum is used for wall plaster by the addition of a filler material such as asbestos, wood pulp, or sand.

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• Calcination of Gypsum

• The temperature is raised to about 120 to 150 oC, with constant agitation to maintain a uniform temperature. In this case, the material in the kettle is known as first-settle-plaster, and it can be sold at this point.

• or it may be heated further to 190 oC to give a material known as second-settle-plaster.

• First-settle-plaster: the half hydrate CaSO4.1/2H2O

• Second-settle-plaster: Anhydrous

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• Hardening of plaster

• The hardening of plaster is essentially a hydration chemical conversion as represented by the equation.

• CaSO4.1/2H2O + 3/2H2O CaSO4.2H2OH= - 2.9 kj

• This is the reverse reaction of dehydration reaction.

• Hydration with liquid water takes place at temperature below about 99 oC and thus gypsum must be heated above 99 oC for practical dehydration.

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• MISCELLANEOUS CALCIUM COMPOUND

• Calcium Carbonate: Calcium carbonate is very widely used industrial chemical, in both its pure and impure state.

• As marble size: as a filler for artificial stone.• for the neutralization of acids

• Marble dust: as abbrasive and in soaps.

• Crude and pulvarized: to sweeten soils in large tonnes

• After wet grinding and levigating: as whitting• and in the paint , rubber and paper industry

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• Calcium Sulfide

• Calcium sulfide is produced by reducing calcium sulfate with coke.

• Using area:

• as depilatory in tanning industry and incosmetics.

• Polysulfides such as CaS2 and CaS5, made by heating sulfur and calcium hydroxide, are consumed as fungicides.

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• Halide salts

• Calcium chloride is obtained commercially as a by-products of chemical manufacture and from natural brines which contains more or less magnesium chloride.

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• Using area:

• to lay dust on highway (because it is deliquescent and remain moist) to melt ice and snow on highways in winter and as antifreeze for concrete.

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• Calcium bromide and iodide

• is produced by action of the halogen acids on calcium oxide or calcium carbonate.

• using area:• for use in medicine and

photography (in hexahydrate form)

• and swimming pool sanitation.

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• Calcium Arsenate

• Calcium arsenate is produced by the action of calcium chloride, calcium hydroxide and sodium arsenate or lime and arsenic acid.

• 2CaCl2 (aq) + Ca(OH)2(s) + 2 Na2HAsO4(aq) Ca3(AsO4)2(s) + 4NaCl(aq)+2H2O(l)

H= -27.8 kj• Using area:• as an insecticides and as fungicides

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• Calcium Organic Compounds

• Calcium acetate and lactate are prepared by the reaction of calcium carbonate or hydroxide with acetic or lactic acid.

• Calcium acetate is formerly pyrolyzed in large amount to produce acetone, but now is used in dying of textiles

• Calcium lactate is sold for use in medicine and foods as a source of calcium.

• Calcium soaps such as stearate, palmitate are made by the action of sodium salts of the acids on a soluble calcium salt such as the chloride. These soaps are insoluble in water but are soluble in hydrocarbons.

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• MAGNESIUM COMPOUNDS

• Magnesium is one of the most widely distributed elements, occupying 1.9 % of the earth's crust.

• It is found in the form of• - chloride• - silicate• - hydrated oxide• - sulfate• - carbonate

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• Raw material and uses

• Important domestic sources of the magnesium salts are

• - seawater• - dolomite• - magnesite (MgCO3)• Using area:• - Extensively for refractories and insulating • - manufacture of rubber, printing

inks,pharmaceuticals and toilet goods• - air pollution control systems for the

removal of sulfur dioxide from stack gases

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• Manufacture• The manufacture of magnesium compounds

by seperation from aqueous solutions • may be divided into four processes:• 1. Manufacture from seawater without

evaporation ,• - using seawater and lime as the principal

raw materials. Magnesium hydrate is dissolved in 10 % hydrochloric acid to furnish a solution of magnesium chloride

• - Concentration of aqueous solution in direct-fired evaporators and producing 76 % magnesium chloride

• - Electrolytic processing of magnesium chloride to produce metallic magnesium

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• 2. Manufacture from bitterns of mother liquors from the solar evaporation of seawater for salt

• 3. Manufacture from dolomite and seawater

• 4. Manufacture from deep-well brines • - Brines (20.7 % CaCl2, 3.9 % CaCl2,

3.9 % MgCl2 and 5.73 NaCl) is precipitated by calcined dolomite to produce a slurry containing 45 % Mg(OH)2 and this is calcinated at high temperatures to produce MgO.

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• Success in obtaining of magnesium compounds form seawater depends upon:

• - means to soften the seawater cheaply, generally with lime or calcined dolomite

• - preparation of purified lime or calcined dolomite slurry

• - economical removal of the precipitated hydroxide from large volume of water

• - purification of the hydrous precipitates

• - development of means to filter the slimes

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• The reactions occurs

• MgCl2 (aq) + Ca(OH)2 (s) Mg(OH)2 (s) + CaCl2 (aq)

H= + 9.46 kJ

• MgSO4 (aq) + Ca(OH)2 (s) + 2H2O (l) Mg(OH)2(s)+CaSO4.2H2O(s)

H= -13.3 kJ

• it is possible to produce of magnesium compound in this way because of

• insolubility of magnesium hydroxide in water.

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• If calcined dolomite uses instead of calcium carbonate;

• the reactions;

• Calcination of dolomite :

• 2CaMg(CO3)2 (s) 2CaO(s) +2MgO(s)+4CO2 (g) H= + 610.9 kJ

• • Slaking :

• 2CaO (s) + 2MgO (s) + 4H2O (l) 2Ca(OH)2 (s) + 2Mg(OH)2 (s) H= -168 kJ

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• Precipitation:

•

• 2Ca(OH)2(s)+2Mg(OH)2 (s) + MgCl2 (aq)+MgSO4 (aq) +H2O (l) 4Mg(OH)2 (s) + CaCl2 (aq) + CaSO4.2H2O (s)

H= -22.6 kJ

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• Calcination:

• 4Mg(OH)2 (s) 4MgO (s) + H2O (g) H= +248.3 kJ

• Hydrochlorination:

• Mg(OH)2(s) + HCl(aq) MgCl2(aq) + 2H2O (l)

H= + 44.7 kJ• 7 % of calcined dolomite is needed for

softening the saewater and the rest is used for precipitating crystalline Mg(OH)2 and then filtered and washed.

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• Miscellaneous magnesium compounds

• Magnesium Carbonates

• dense MgCO3 is used in magnesite brick to the low density 4MgCO3.Mg(OH)2.5H2O and 3MgCO3.Mg(OH)2.3H2O

• using area:• as fillers in inks, paints and varnishes

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• Oxide and Hydroxide of Magnesium• • On heating magnesium carbonate or

hydroxide , magnesium oxide (MgO) is formed.

• using area:

• in the vulcanization of rubber , material for making other magnesium compounds, as an insulating materials, as a refractory material, as an abrasive.

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• Magnesium peroxide

• the product of reaction between magnesium sulfate and barium peroxide

• and is used as an antiseptic and a bleaching agent.

• Magnesium Sulfate

• is prepared by the action of sulfiric acid on magnesium carbonate or hydroxide

• It is sold in many form one of which is the hydrate MgSO4. 7H2O (Epsom salt)

• using area:• extensively as sizing and as a fireproofing

agent.

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• Magnesium Chloride

• is produced from hydrochloric acid and magnesium hydroxide

• using area:

• in ceramics, in the sizing of paper, and in the manufacture of oxychloride cement and making of metallic magnesium

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• Magnesium Silicates

• Two widely used naturally occuring compounds , asbestos and talc.

• Asbestos is a magnesium silicate mixed with varying quantities of silicate of calcium and iron. This is used for manufacturing of fireproof and insulating materials.

• But it has cancercausing chracteristics of its fibers.• Talc is a rather pure magnesium silicate in the form

of 3MgO.4SiO2.H2O found naturally in soapstone.

• using area:

• as a filler in paper and plastics and in many cosmetic and toilet preparation.

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